Silica fume has been known as an additive in cements since the 1980s. Since then the extent of its use has grown and it is now generally regarded as an indispensable material in the production of high performance concrete (HPC), particularly ultra high performance concrete (UHPC) if properties, such as compressive strength, adequate for use in modern construction methods are to be secured.
High performance concrete generally has a compressive strength at 28 days of 50 to 100 MPa. Ultra high performance concrete generally has a compressive strength at 28 days greater than 100 MPa and generally greater than 120 MPa.
Silica fume, also known as micro-silica, is a by-product in the production of silicon or ferrosilicon alloys. Its main constituent is amorphous silicon dioxide. Individual particles generally have a diameter of about 5 to 10 nm. Individual particles agglomerate to form 0.1 to 1 μm agglomerates and then aggregate together into 20 to 30 μm aggregates. Silica fume generally has a BET surface area of 10-30 m2/g. It is known as a reactive pozzolanic material.
A pozzolan is described in Lea's Chemistry of Cement and Concrete, 4th edition, published by Arnold as an inorganic material, natural or synthetic, which hardens in water when mixed with calcium hydroxide (lime) or with a material which can release calcium hydroxide (such as Portland Cement clinker). A pozzolan is generally a siliceous or siliceous and aluminous material which, alone, possesses little or no cementitious value but which is capable, in the presence of moisture, of reacting chemically with calcium hydroxide at ambient temperature to form compounds having cementitious properties.
It will be understood, therefore, that silica fume takes an active part in the setting process of concrete mixes containing it and is actively involved in the formation of cementitious compounds which bind together the various particulate materials present and thereby contribute to the strength of the resulting concrete.
WO 2005/077857 describes a very high performance concrete which comprises a mixture of calcined bauxite sands of different granulometries and silica fume in which ultra fine calcium carbonate having defined specific surface values (greater than 10 m2/g) and form index (at least 0.3, where the form index is the ratio of particle thickness to particle length, measured by field effect scanning electron microscopy) is added in order to improve the whiteness of the concrete. The mean diameter of the ultra-fine particles of calcium carbonate is about 70 nm.
WO 2006/134080 A1 describes the use of calcium carbonate particles in the production of construction materials such as plaster, stucco, cement render, mortar and concrete having improved properties. The improved properties described include good sound absorption properties and high abrasion resistance but do not include compression resistance.